RAID 0 is a mirror - it does not double the write speed (impossible - as it's actually writing the data to two discs (albeit simultaneously) it's marginally slower). Thoretically it can double read speed as it can get the data from either disc - with a traditional HDD it can get the data from whichever disc's heads are physically closer. In practice speed is about 1.2x. If the disc is solid state then the performance increase is negligeable.

The throughput of a PCIx is better assuming there is enough buffering.